1. Field of the Invention
The present invention relates to an anode element with improved performance, a method of manufacturing the same, and a solid electrolytic capacitor having an improved withstand voltage by utilizing the anode element.
2. Description of the Background Art
Generally, an anode element obtained by coating a surface of a metal with an anodic oxide film is used as an anode element of a solid electrolytic capacitor. This is because the solid electrolytic capacitor employing the anodic oxide film as a dielectric is relatively inexpensive and its capacity is great. Examples of the solid electrolytic capacitor widely used in present days include a wound-type structure (see Japanese Patent Laying-Open No. 11-074155), a single-plate-type structure (see Japanese Patent Laying-Open No. 11-329900), and the like. The anodic oxide film is normally formed by subjecting the metal to formation treatment using anodic oxidation. The anodic oxide film is mainly composed of an oxide of the metal.
A withstand voltage of the solid electrolytic capacitor is considered to be dependent on a thickness of the anodic oxide film, and it is considered necessary to form an anodic oxide film having a large thickness in order to increase the withstand voltage. Accordingly, a formation voltage in formation treatment using anodic oxidation described above is applied in proportion to the withstand voltage of the solid electrolytic capacitor. More specifically, the formation voltage is set to magnitude two to three times as great as the withstand voltage of the solid electrolytic capacitor so as to form the anodic oxide film. Here, for example, EIAJ RC-2364A (Japan Electronics and Information Technology Industries Association Standards (revised March, 1999)) is available as a method of measuring a withstand voltage of the anodic oxide film. In the following, the withstand voltage of the dielectric on the surface of the metal in accordance with EIAJ RC-2364A is referred to as a Vt withstand voltage.
The anodic oxide film is formed, for example, through anodic oxidation by applying a formation voltage to the metal while the metal is immersed in an aqueous solution containing ammonium adipate. Generally, it is considered that close attention should be paid for forming a uniform anodic oxide film on the entire metal surface, without forming a porous film. Namely, the anodic oxide film is of a barrier-type, that is, the surface of the metal is completely coated to a uniform thickness. This is because the anodic oxide film of the barrier-type attains a Vt withstand voltage higher than a porous anodic oxide film on the condition that the anodic oxide film of the barrier-type and the porous anodic oxide film are identical in the thickness.
In addition, it is considered that a leakage current of the solid electrolytic capacitor and a leakage current in liquid during the anodic oxidation step in forming the anodic oxide film in the solid electrolytic capacitor correlate with each other. Therefore, in order to prevent the leakage current in liquid in anodic oxidation described above, complicated fabrication steps such as heat treatment before or during anodic oxidation have been adopted. Even if the Vt withstand voltage is increased by adopting the method above, however, the withstand voltage of the solid electrolytic capacitor is not raised as expected. Namely, large electric power is consumed for obtaining a desired withstand voltage of the solid electrolytic capacitor.
In addition, if the anode element is cut and worked to form the solid electrolytic capacitor, burdensome steps such as anodic oxidation, washing, drying, and the like are required for formation re-treatment for repairing an end face of the cut edge. Further, even if repairing as such is performed, the Vt withstand voltage of the repaired anodic oxide film cannot be equivalent to the Vt withstand voltage of the anodic oxide film before cutting and working. Such non-uniformity of the repaired anodic oxide film becomes a factor leading to deterioration of the withstand voltage of the solid electrolytic capacitor, higher leakage current, and resultant lower reliability of the solid electrolytic capacitor. In addition, if a voltage equal to or greater than the formation voltage used for forming the anodic oxide film in fabricating the anode element before cutting and working is applied at the time of formation re-treatment, the anodic oxide film is broken. Therefore, such application of voltage not smaller than the Vt withstand voltage at the time of formation re-treatment has conventionally been avoided. In particular, if the solid electrolytic capacitor is of the wound-type, damage to the anodic oxide film due to winding of the anode element is greater as the withstand voltage of the solid electrolytic capacitor is greater, and reaction takes place in a gap where liquid circulation is insufficient in the formation re-treatment, which results in the problem of corrosion of the anode element, corrosion of a lead due to generated air bubbles, or the like.
Here, from a viewpoint of increasing capacitance of the solid electrolytic capacitor by improving a method of forming an anodic oxide film, a method allowing increase in capacitance even with a withstand voltage of the solid electrolytic capacitor not greater than 150V, by forming an anodic oxide film after hydration treatment and heat treatment of a metal, has been studied (see Japanese Patent Laying-Open No. 08-241832).